In the medical field, while performing surgery on a patient, the surgeon often operates on complex anatomical structures composed of multiple elements, for example on a bone structure, composed of a bone matrix which contains mineral elements and the bone marrow where blood cells are formed, or an intervertebral disc composed of a matrix of fibrous or gelatinous cartilage and water.
It is useful for the surgeon to be able to evaluate the quality of these structures objectively.
In particular, when the surgery on a bone structure consists of inserting an implant, for example a screw in a vertebra or a nail in a femur, the holding of this implant greatly depends on the quality of the anchorage and therefore on the quality of the bone structure into which it is inserted. An implant installed in a bone structure of poor quality, particularly porous bone, will not hold as well as the same implant installed in a bone structure of good quality.
After the operation, good anchorage of the implant is crucial to better union, to better fusion, and especially to decreasing the risks associated with poor anchorage of the implant in the bone structure, for example such as the onset of pain-generating pseudoarthrosis or the need for new surgery if implant migration occurs.
Also, with the aging of the general population, the trend is toward treating increasingly elderly patients who therefore have less solid bones. New implants are appearing on the market, such as expansion screws, dedicated for use in fragile bone not having bone structure of good quality. If the surgeon has objective knowledge of the quality of the bone structure he will be working with, he can adapt his therapeutic method accordingly. For example, for screw implantation in a vertebra, if the bone structure is not of good quality, he may decide to choose a screw of a larger diameter, or to use a specific implant, or to inject consolidating cement, or to treat an additional vertebra, or to use any other method he deems appropriate.
There are known techniques used prior to surgery that provide the surgeon with an indication of the bone structure quality.
For example, the DEXA method (Dual Energy X-ray Absorptiometry) allows measuring the bone density of different regions of the skeleton. This involves measuring the attenuation of two x-ray beams of different energy levels through tissues (soft and hard: organs and bones). Once the attenuation is known, the density of the traversed tissues is deduced using an attenuation equation according to the Beer-Lambert law. The examined areas may be the entire body or portions of it, specifically the spinal column, the hip, the neck of the femur, and the forearm (radius). The results of this examination are expressed as a score describing the measured bone mineral density in comparison to the corresponding normal value for the area in question. This examination allows evaluating the risk of fractures even before a first fracture appears. It is currently considered to be the standard for testing older women for osteoporosis.
Also known, from application WO 2008/119992, is a technique which first consists of applying and measuring alternating electrical signals emitted over a wide frequency range, within a portion of the body, then processing the electrical signals to determine the impedance and the phase shift for each of the frequencies and to use this to deduce a bone density value for that part of the body.
These non-invasive and painless techniques which are based on a rapid examination offering good results in terms of precision and reproducibility have the disadvantage, however, of providing a general examination which analyzes both the high-density cortical bone and the low-density cancellous bone.
As there may be a large variation within a bone structure in terms of quality (for example one vertebra may be of lesser quality than another neighboring vertebra in the same patient), knowing the general quality of the bone structure is insufficient. Local information is required.
To obtain local information on the quality of the bone structure, there is the known technique of bone biopsy, which consists of collecting a sample, generally under anesthesia, of a small bone fragment in order to analyze its structure. In principle, the biopsy location is the hip bone (iliac bone).
However, this technique has a certain number of disadvantages, including the need to expose the patient to an additional painful procedure and the inability to extrapolate this to the entire region where the surgeon is to work. In addition, obtaining information on the quality of the bone structure requires processing the collected fragment and is therefore not immediate.
Aside from the disadvantages mentioned above, the above techniques are pre-operative and increase the number and duration of the procedures the patient must undergo.
During surgery, however, it is critical that the operating time be as short as possible in order to decrease the risks (anesthesia, infection, etc.). As a result, the measurement of the local quality of the bone structure must be obtained without increasing the operating time.
Also known, from document U.S. Pat. No. 6,997,883, is a dental diagnostic system. The system comprises a body adapted for insertion into a preformed hole in a tooth, and a resistance measuring device comprising two electrodes for distinguishing healthy tooth tissue from diseased tooth tissue based on a difference in conductivity resulting from a difference from filling the tooth with liquid. This system, which applies to a tooth and not to a bone structure, requires drilling the tooth and filling the tooth with a liquid beforehand. In addition, the quality of the tissue is determined in a relative manner based on a variation in conductivity.
Document WO 2009/152244 describes a system for determining the dimensions of a hole in a bone structure, also comprising a body adapted for insertion into a preformed hole.
In the two systems mentioned above, the electrodes are in contact with tissue which is not the intact bone for which the quality is to be measured, but with a material resulting from mixing bone debris created from the prior drilling and the fluids that are present, for example blood. However, the resulting mixture does not have the same quality as the intact bone prior to drilling. As a result, these systems do not allow continuously determining the quality of a bone structure, particularly while drilling the bone structure.
A need exists for a system for determining the quality of the bone structure which allows the surgeon to be able to evaluate the quality of this structure objectively, locally, directly in contact with the bone, in real time and continuously while drilling the bone structure.